Yankee dryer cylinder operating without steam

ABSTRACT

A Yankee dryer cylinder comprising a fixed hollow shaft provided with an inlet mouth through which a flow of hot air is fed into the cavity, and an outlet mouth through which the flow of hot air exits the dryer cylinder. The Yankee dryer cylinder provides, furthermore, a cylindrical shell having an internal surface and rotatably mounted about the fixed hollow shaft. Into the cavity a partition wall is provided which divides the fixed hollow shaft in a feeding portion and in a discharging portion.

FIELD OF THE INVENTION

The present invention relates to the field of machines for producing paper and similar products and, in particular, relates to an improved structure of dryer cylinder, also known as Yankee dryer cylinder, or simply Yankee cylinder.

STATE OF THE ART

As known, the plants for producing paper provide the use of a headbox for distributing a mixture of cellulosic fibres and water on a forming fabric, and, in some cases, additives of different kinds. In this way, a determined amount of water is drained and, then, the dry content of the mixture layer arranged on the forming fabric is increased.

The content of water is, then, reduced, through a sequence of passages among many fabrics and/or felts of the mixture layer, up to reach a consistency such that allows to pass through a drying section. This usually comprises at least a dryer cylinder, also called “Yankee dryer cylinder”, or simply “Yankee cylinder” and a drying hood that is fed with hot air. In particular, the web of treated humid paper is laid on the external surface of the Yankee cylinder, while the inside of the Yankee cylinder is heated, for example, by introducing steam.

Because of the steam produced inside the Yankee cylinder and of the hot air, which is blown by the hood on the paper, the web of humid paper, which is laid on the external surface, is gradually dried. When the desired drying level is achieved, the web of paper is removed from the external surface of the Yankee cylinder by means of a blade, or a doctor blade, or by tensioning, depending on the desired product, and, in particular, crepe paper, or smooth paper.

A Yankee cylinder comprises essentially two heads, or end walls, between which a cylindrical shell is positioned. Generally, a bearing journal, which is mounted, in operating conditions, on a respective bearing, is fixed to each head. A hollow shaft is mounted inside the shell. The heads and/or the shell are provided with inspection apertures through which at least a worker enters into the cylinder for carrying out, periodically, ordinary, or extraordinary maintenance interventions.

The constituent elements of the Yankee cylinder, i.e. the heads, the shell, the bearing journals etc. can be obtained by iron casting and can be fixed by means of bolts.

Alternatively, the Yankee cylinders can be made of steel. In this case the two heads can be fixed to the cylindrical shell by means of screw bolts, or more frequently by means of weld beads.

Both in the Yankee cylinders made of cast iron and in those made of steel, the cylindrical shell has an internal surface provided with circumferential grooves. These are adapted to collect the condensate formed for the transfer, toward outside, of the latent heat of vaporization from the steam that has been introduced inside the Yankee cylinder.

However, the Yankee cylinders that use steam as source of thermal energy have many drawbacks.

Firstly, the great amount of steam that are necessary in these types of machines involves high running costs. Furthermore, it is necessary to use specific apparatuses both for producing steam and for transporting the steam same to the Yankee cylinder.

In addition to what described above, in order to satisfy the safety standards provided for the apparatuses, which use pressure steam, it is necessary to adopt complicated structural solutions, in particular in order to be able to resist to the high stresses, to which the different parts of the dryer cylinder are subjected, mainly thermo-elastic stresses, caused by the high temperature of the steam that is introduced inside dryer cylinder.

Other solutions of the state of the art provide, instead, to feed a flow of air into the cylinder, the air is then burned by one, or more burners for obtaining the necessary thermal energy. An example of Yankee cylinder using air is described in EP0708301A1.

In particular, the solution described in this document provides a central duct and a shell rotating about a shaft. The combustion air is introduced through the duct and fed to some burners arranged, in determined positions, within the cylinder. The burners, therefore, produce the heat that hits the shell. The exhaust fumes exit the central duct, at the opposite side of the inlet of air.

However, the solution described in EP0708301A1 does not appear to be particularly advantageous. In fact, it is necessary to allocate not a little space, to the housings of the burners, of the air feed ducts, and of the discharging ducts of the burnt gases. This considerably decreases the surface available for the heat exchange, thus reducing the efficiency of the machine.

Furthermore, the solution described in EP0708301A1, in particular, owing to the presence of the burners and the ducts, does not seem to be able to uniformly distribute the heat produced on the shell surface. This thing negatively affects the quality of the final product.

A further drawback of the solution described in EP0708301 is that the internal volume of the cylinder can become saturated with inflammable gases and, therefore, the flame, which is produced by the internal burners, can cause explosions and, therefore, exposing the workers to risks, besides compromising the efficacy of the plant.

A further solution is described in U.S. Pat. No. 3,633,662. However, also this construction solution is complex and is not able to guarantee a uniform heat distribution at the internal surface of the shell.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a Yankee dryer cylinder that is able to solve the above disclosed drawbacks of the Yankee cylinders of prior art, and, in particular, that is able to provide a uniform heat distribution at the internal surface of the shell and, therefore, to produce a final product of high quality.

It is also an object of the invention to provide a Yankee dryer cylinder that is able to considerably reduce the running costs of the plant where it is installed.

It is a further object of the invention to provide a Yankee dryer cylinder that has a simpler construction and is less expensive than the Yankee cylinders of prior art.

This and other objects are achieved by a Yankee dryer cylinder, according to the present invention, comprising:

-   -   a fixed hollow shaft having a cavity and a lateral surface         provided with a plurality of holes arranged to put in         communication said cavity with the external environment, said         fixed hollow shaft being provided with:         -   an inlet mouth through which a flow of hot gas having a             predetermined temperature, in particular a temperature             higher than 100° C., is arranged to be fed into said cavity;         -   an outlet mouth through which said flow of hot gas is             arranged to flow outside said dryer cylinder;     -   a cylindrical shell provided with an internal surface and         rotatably mounted about said fixed hollow shaft, between the         internal surface of said cylindrical shell and said lateral         surface of said hollow shaft, an internal chamber being defined;     -   whose main characteristic is to provide, furthermore:     -   a partition wall mounted within said cavity in such a way to         divide said fixed hollow shaft in a feeding portion and in a         discharging portion, said feeding portion provided with at least         one feeding hole arranged to feed said flow of hot gas, from         said cavity to said internal chamber, in such a way to hit said         internal surface, said discharging portion provided with at         least one discharging hole, which is pneumatically connected to         said outlet mouth, said flow of hot gas, after having hit said         internal surface of said shell, arranged to pass through said,         or each, discharging hole from said internal chamber to said         cavity, in such a way to exit said Yankee dryer cylinder through         said outlet mouth. In particular, the hot gas can be a mixture         of gases.

Advantageously, the mixture of gases can be, in particular, air.

Advantageously, the temperature of the flow of hot gas can be set between 100° C. and 750° C., advantageously set between 250 and 700° C., preferably set between 300° C. and 600° C.

Alternatively, the flow of hot gas can be a flow of combustion fumes having a temperature higher than 100° C.

In particular, a feeding device is provided for feeding a flow of hot air, said feeding device arranged to feed said the flow of hot air into said feeding portion of said cavity through said inlet mouth.

In a provided embodiment, downstream of said Yankee dryer cylinder, a suction device is provided that is connected to the outlet mouth of the fixed hollow shaft. In particular, the suction device is arranged to suck the flow of hot gas that is fed into the cavity of the fixed hollow shaft through the inlet mouth. In this way it is possible to assist the exit of the gas from the dryer cylinder.

Advantageously, the internal surface of the cylindrical shell provides a plurality of circumferential grooves.

Preferably, a plurality of distributor members is provided in the internal chamber arranged to distribute the flow of hot air, which comes from said cavity of said fixed hollow shaft, on the internal surface of the shell.

In particular, each distributor member of the above disclosed plurality is arranged to distribute a share of flow of hot air fed into the cavity of the fixed hollow shaft, on a respective determined portion of the internal surface of the shell.

More in detail, the distributor members are configured in such a way to define, in the internal chamber, a predetermined path for the hot air between the cavity of the feeding portion and the cavity of the discharging portion.

In particular, the Yankee cylinder and the fixed hollow shaft are arranged coaxially to each other.

In an embodiment according to the invention, each distributor member is tubular-shaped, and it is provided with at least one inlet aperture facing towards at least one feeding hole of the lateral surface of the fixed hollow shaft, and at least one distribution aperture facing towards the internal surface of the cylindrical shell.

Analogously, the feeding aperture can be associated to a feeding panel provided with a plurality of inlet holes. This technical solution, in particular, is adopted in the case in which the feeding panel is arranged in front of a plurality of suction holes.

According to preferred embodiment, the above disclosed plurality of distributor members is integral to the fixed hollow shaft.

Advantageously, each distributor member is arranged in front of a predetermined number of suction holes.

In particular, the feeding portion is provided with a plurality of feeding holes and the discharging portion is provided with a plurality of discharging holes. In this case, both the feeding holes and the discharging holes are advantageously arranged on respective rows.

In particular, the plurality of feeding holes and the plurality of discharging holes are distributed along all the perimeter of the fixed hollow shaft.

For example, the rows, along which, the feeding holes, and the discharging holes are distributed can be oriented longitudinally to the fixed hollow shaft.

Advantageously, each distributor member can be oriented such that the respective feeding aperture is positioned on the respective row of the suction holes.

Preferably, between the fixed hollow shaft and the Yankee cylinder, at least a first and a second bearing are mounted that are arranged at opposite end portions.

According to another aspect of the invention, a plant for drying a humid web of paper, or similar product, comprises:

-   -   a Yankee dryer cylinder as above described, said Yankee dryer         cylinder provided with an external surface on which, in use, the         humid web to be dried is arranged;     -   a drying hood arranged, in use, at the opposite side of said         Yankee dryer cylinder with respect to said humid web, said         drying hood arranged to cause the drying of said humid web         operating in combination with said Yankee dryer cylinder;     -   a duct arranged to pneumatically connect said drying hood and         said cavity of said fixed hollow shaft, said duct being arranged         to feed a flow of hot air circulating within said drying hood,         within said cavity.

According to a further aspect of the invention, a method for drying a humid web of paper, or similar product provides the steps of:

-   -   disposing of a fixed hollow shaft having a cavity and a lateral         surface provided with a plurality of holes arranged to put said         cavity in connection with the external environment, said fixed         hollow shaft being provided with:         -   an inlet mouth through which a flow of hot air is arranged             to be fed into said cavity;         -   an outlet mouth through which said flow of hot air is             arranged to flow outside said dryer cylinder;     -   disposing of a cylindrical shell provided with an internal         surface and rotatably mounted about said fixed hollow shaft,         between the internal surface of said cylindrical shell, and said         lateral surface of said hollow shaft, an internal chamber being         defined;     -   disposing of a partition wall mounted within said cavity in such         a way to divide said fixed hollow shaft in a feeding portion and         in a discharging portion;     -   feeding a flow of hot air into said feeding portion, said flow         of hot air passing from said cavity to said internal chamber         through at least one feeding hole provided in said feeding         portion, then, hitting said internal surface, and, then, passing         in said discharging portion of said fixed hollow shaft through         at least one discharging hole made in said discharging portion,         and, then, exiting said cylinder through said outlet mouth, said         flow of hot air, after having hit said internal surface of said         shell, being arranged to pass through said, or each, discharging         hole, from said internal chamber to said cavity, in such a way         to flow outside said Yankee dryer cylinder through said outlet         mouth.

Other features of the invention are described in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be now shown with the following description of its exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings in which:

FIG. 1 shows a perspective view, partially sectioned, of a Yankee dryer cylinder according to the invention;

FIG. 2 shows the dryer cylinder of FIG. 1 according to a longitudinal section;

FIG. 3A shows an enlargement of a part of the distribution mouth of a distributor member, in order to highlight some details, in particular the distribution panel that is provided;

FIG. 3B shows an alternative embodiment of the distribution panel of FIG. 3A;

FIG. 4 shows the dryer cylinder, according to the invention, without the heads, in a perspective elevational side view;

FIG. 5 diagrammatically shows, in a perspective elevational side view, the fixed hollow shaft in running conditions;

FIG. 6 shows the fixed hollow shaft of FIG. 5, sectioned along a longitudinal plane;

FIG. 7 shows the dryer cylinder, according to the invention, without the heads, in a perspective elevational side view different from that of FIG. 5;

FIG. 8 diagrammatically shows an enlargement of a distribution panel provided by the invention at the distribution mouth;

FIG. 9 shows in a perspective view, in longitudinal section, an alternative embodiment of the fixed hollow shaft of FIG. 6;

FIG. 10 shows a longitudinal view of a first embodiment of the Yankee cylinder equipped with the alternative embodiment of the fixed hollow shaft of FIG. 9;

FIG. 11 shows a longitudinal view of the alternative embodiment of the Yankee cylinder of FIG. 10 equipped with the alternative embodiment of the fixed hollow shaft of FIG. 9;

FIG. 12 shows a perspective view of a further alternative embodiment of the fixed hollow shaft of FIG. 6;

FIG. 13 shows a cross sectional view of the fixed hollow shaft of FIG. 12;

FIG. 14 shows in detail an enlargement of a portion of FIG. 13 in order to highlight some features of the invention;

FIGS. 15A and 15B show two possible alternative embodiments of a plant for drying a humid web of paper, according to the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

As diagrammatically shown in FIG. 1 a structure of Yankee dryer cylinder 1, according to the present invention, comprises a cylindrical shell 10 having a internal surface 11. Furthermore, a first and a second heads, or end walls, 71 and 72, are provided that are fixed at the opposite ends of the cylindrical shell 10. This latter is rotatably mounted about a fixed hollow shaft 20 having a cavity 21 and a lateral surface 22 provided with a plurality of holes 25. The fixed hollow shaft 20 is provided, in particular, with an inlet mouth 23, through which a flow of hot gas 100, preferably a flow of hot air, in particular having a temperature higher than 100° C., is fed into the cavity 21. The temperature of the hot air is advantageously less than 700° C. in order to avoid to generate too high thermoelastic stresses on the structure of cylinder 1.

The fixed hollow shaft 20 is, then, provided with an outlet mouth 24 provided at the opposite side of inlet mouth 23. Through the outlet mouth 24, the flow of hot air 100, which is introduced through the inlet mouth 23, exits the dryer cylinder 1 after having hit the internal surface 11 of shell 10.

According to the invention, into the cavity 21, a partition wall 35 is provided mounted in such a way to divide the fixed hollow body 20 in a feeding portion 20 a, arranged upstream of the partition wall 35, and in a discharging portion 20 b, arranged downstream of the partition wall 35 (FIG. 6).

The flow of hot air 100 that is fed into the cavity 21 of the feeding portion 20 a, is arranged to pass, through a plurality of feeding holes 25 a, into an internal chamber 30 defined between the internal surface 11 of cylindrical shell 10 and the lateral surface 22 of the hollow shaft 20. The flow of hot air 100 hits, therefore, the internal surface 11 and, then, enters the discharging portion 21 b of cavity 21 through a plurality of discharging holes 25 b. These are, in particular, closer to the outlet mouth 22 than the feeding holes 25 a.

In the embodiments shown, for example in the FIGS. 1, 2 and 6, the partition wall 35 is arranged transversely to the fixed hollow shaft 20, in particular, it is substantially orthogonal to the longitudinal axis 102 of the fixed hollow shaft 20. In this case, therefore, the feeding portion 20 a is closer to the inlet mouth 23, and the discharging portion 20 b is closer to the outlet mouth 24.

In the alternative embodiments shown in the FIGS. 9 and 10, the partition wall 35 is, instead, oriented longitudinally to the fixed hollow body 20. In this case, the feeding portion 20 a is positioned in the upper part of the fixed hollow shaft 20, while the discharging portion 20 b is arranged in the lowest part.

As shown, for example in FIG. 1, the Yankee dryer cylinder 1 provides a plurality of distributor members 40 housed within the internal chamber 30. More in detail, each distributor member 40 is adapted to distribute a share of the flow of hot air 100 coming from the feeding portion 21 a of cavity 21, on a respective determined portion of the internal surface 11 of shell 10.

As shown in detail in the FIGS. 4 and 6, the distributor members 40 can be oriented radially to the Yankee cylinder 10. In particular, each distributor member 40 can extend longitudinally to the Yankee cylinder 10.

In the alternative embodiment diagrammatically shown, as an example, in the FIGS. 4 and 7, each distributor member 40 is tubular-shaped, for example parallelepiped-shaped. In particular, each distributor member 40 can provide at least one inlet aperture 41 facing towards at least a feeding hole 25 a of the feeding portion 20 a of fixed hollow shaft 20, and at least one distribution aperture 42 facing towards the internal surface 11 of the cylindrical shell 10.

As it is shown in detail in FIG. 3A, the distribution aperture 42 of the distributor member 40 is advantageously associated to a distribution panel 45 provided with a plurality of distribution holes 46. These can be uniformly distributed on the surface of the distribution panel 45, in such a way to uniformly distribute the share of the flow of hot air 100, which is delivered by the distributor member 40, on a respective portion of the internal surface 11 of cylindrical shell 10. In the alternative embodiment shown in FIG. 3B, the distribution holes 46 of the distribution panel 45 are frustoconical-shaped. In this way, it is possible to reduce the pressure drop in the holes, and, therefore, to increase the flow of hot air hitting the shell 10.

It is appropriate to specify that, in FIG. 3, the internal surface 11 of shell 10 is shown provided with a plurality of circumferential grooves 15, only as an example. In fact, according to the present invention, the internal surface 11 of shell 10 is not necessarily provided with circumferential grooves. The distribution panel 45 can be a distribution plate made of a stiff material. Alternatively, the distribution panel 45 can be made of a porous material, for example a woven fabric, or a nonwoven fabric, or similar materials, provided of a predetermined level of porosity. In this case, the pores of the material of which the panel is made, act as distribution holes.

As shown, for example in FIG. 7, the distributor members 40 are arranged at predetermined distance from each other and the distribution apertures 42 are arranged at a predetermined distance from the internal surface 11 of shell 10. More in detail, space 50, which is defined by the lateral surfaces facing each other 43 and 44 of two next distributor members 40, in use, is passed through by an exiting flow of hot air. In fact, the flow of hot air coming from cavity 21 of the fixed hollow shaft 20, once distributed by each distributor member 40 on a corresponding portion of internal surface of the cylindrical shell 10, is forced to pass into the space 50, which is arranged downstream of the distributor member 40, from which it comes from, in particular owing to the rotational motion of cylindrical shell 10. Once the flow of hot air 100 has transferred its heat at the internal surface 11 of shell 10, it becomes cold and passes through the space, which is present between two next distributor members 40, going towards the discharging holes 25 b of the fixed hollow shaft 20. The flow of hot air 100 passes, therefore, from the above described space 50, at first, in the cavity 21, through the discharging holes 25 b and, then, outside of the fixed hollow shaft 20, through the outlet mouth 24. This passage can be caused by a suction device, that is not shown in figure for reasons of simplicity, arranged downstream of the outlet mouth 24 of the fixed hollow shaft 20.

In the preferred embodiment shown in the figures from 1 to 7, the feeding holes 25 a and the discharging holes 25 b are positioned on respective rows longitudinal to the fixed hollow shaft 20. More precisely, as shown in detail in the FIGS. 5 and 6, the feeding holes 25 a and the discharging holes 25 b are positioned on respective staggered longitudinal rows, i.e. arranged on the external surface of the fixed hollow shaft 21, but at different generatrix, in case of cylindrical geometry of the shaft 20, as shown in the figure, or, however, along different directions, but parallel to the axis of the shell 10, in the case in which the shaft 20 has a different geometry, for example a parallelepiped geometry. In particular, each row of discharging holes 25 b is arranged downstream of a respective row of feeding holes 25 a with respect to the advancing direction of the flow of hot air 100 along the internal surface 11 of shell 10. This is determined by the rotational direction of cylindrical shell 10 about its axis of rotation. In this way, therefore, an obligatory path is obtained for the flow of hot air 100 that is, thus, forced to pass from cavity 21 of the fixed hollow shaft 20, into distributor member 40, through the feeding holes 25 a, e, after having hit the internal surface 11 del cylindrical shell 10, to pass into the space 50 defined between two next distributor members 40.

In the embodiments shown in the figures from 9 to 11, the inlet mouth 23 and the outlet mouth 24 are arranged at the same side of the fixed hollow body 20. In particular, at the end of the fixed hollow shaft 20 that is opposite to the inlet mouth and to the outlet mouth 24, a closing wall 26 is provided. In this way, it is possible to simplify the construction. In fact, as shown in the FIGS. 10 and 11, since there is no further need of using a duct with a great diameter for the exit of hot air from Yankee cylinder 1, it is sufficient a shaft 80 having a small diameter and it is, therefore, possible to use a bearing 62 having a small size.

In the FIGS. 12 and 13 a further alternative embodiment of the fixed hollow shaft 20, provided by the present invention, is shown. In this case, the partition wall 35 extends longitudinally along the fixed hollow shaft 20, but comprises two planar portions 35 a and 35 b inclined of a predetermined angle α. More in detail, the feeding portion 20 a of the fixed hollow shaft 20 having the feeding holes 25 a, is, in this case, set between the two planar portions 35 a and 35 b. The discharging portion 20 b provided with the discharging holes 25 b is the portion of the fixed hollow shaft 20 corresponding to a central angle β, in particular the explementary angle of that angle α. In this case, therefore, the flow of hot air is delivered at the portion 13 of the internal surface 11 of shell 10 of Yankee dryer cylinder 1. The rotational motion of shell 10 at high speed, for example 3 rounds per sec, contributes, then, to distribute, substantially uniformly, the flow of hot air on all the internal surface 11 of shell 10. Also this embodiment can provide one, or more, distributor members 40 having, or not having, the distribution plate 45, as above described. As shown in detail in the enlargement of FIG. 14, the distributor members 40 are arranged at a predetermined distance d from the internal surface 11 of shell 10.

The hot air fed into the cavity 21 of the fixed hollow shaft 20, can be obtained, in particular, as exhaust fumes, by a machine for producing electrical energy, or by means of a cogenerator, i.e. by means of a “turbogenerator”, or a different machine for producing energy that is arranged in the plant where the Yankee dryer cylinder 1, according to the invention, has to be installed, or however in the neighbourhood of the same. If it is necessary to reduce the temperature of the exhaust fumes, normally about 600-700° C., it is sufficient to cool such exhaust fumes before feeding the same to the fixed hollow shaft 20, for example mixing the exhaust fumes with fresh air.

In this way, it is possible to optimize the plant energy consumptions considerably reducing the running costs.

In the FIGS. 15A and 15B, two different embodiments are diagrammatically shown of a plant 200, according to the invention, for drying a humid web of paper 160, or similar product. More in detail, the plant 200 comprises a Yankee dryer cylinder 1, as above described with reference to the FIGS. 1 to 11. The Yankee cylinder 1 is, in particular, provided with an external surface 12, on which, in use, the humid web 160 to be dried is positioned. Plant 200 comprises, furthermore, a drying hood 150 arranged to work in combination with the Yankee cylinder 1 in order to cause the drying of the humid web of paper. Generally, the drying hood 150 is arranged to suck the humidity, which is produced by contact of the humid paper web with the surface 12 of the Yankee dryer cylinder 1 and to feed a flow of hot air on said humid paper web 160. According to the invention, plant 200 provides, furthermore, a duct 180 arranged to pneumatically connect the drying hood 150 to the cavity 21 of the fixed hollow shaft 20. More precisely, the duct 180 is arranged to feed a flow of hot air, which circulates within the drying hood 150, into the cavity 21. This particular construction solution allows to optimize the energy consumption, in particular, for producing hot air.

In fact, as known, within the drying hood 150, which is positioned above the Yankee dryer cylinder 1, circulates a high flow of hot air, in part, produced by the evaporation of the humidity caused by the contact of humid web of paper 160 with the external surface of Yankee cylinder 1, and, in part, produced within the drying hood 150 that is provided with at least one burner, in particular a burner powered by methane gas, for producing hot air. The hot air produced within the hood 150 is directed towards the humid web 160, which slides on the surface of the dryer cylinder, causing the drying of the humidity that is evaporated from the humid paper web 160.

The drying hood 150 is, in particular, divided into a “humid” part 151 and a “dry” part 152. The present invention provides both the case in which the duct 180 is connected to the dry part (FIG. 15A), and the case in which the duct 180 is connected to the humid part of the hood 150 (FIG. 15B). This can be obtained, connecting the duct 180 directly to one of the two parts of the hood 150, as shown in the FIGS. 15A and 15B, or to a respective discharging duct, this case is not shown in the figures for reasons of simplicity.

More precisely, the temperature of the air circulating within the dry part of the drying hood, is about 600-700° C. In the case in which the available hot air has a temperature that is considered to be too high and, therefore, dangerous, for the possibility to generate high thermal stresses in the structure of the Yankee cylinder 1, the hot air exiting the dry part can be mixed with cold air, before introducing the same in the fixed hollow shaft 20.

This problem does not exist, instead, when the air is withdrawn from the humid part of the hood 150. The humid part of the hood is, in particular, passed through by a flow of hot air having a temperature less than the air circulating within the dry part. The temperature of the air circulating within the humid part of hood 150, in fact, is about 300-400° C. and, therefore, ideal for being fed into the cavity 21 of the fixed hollow shaft 20, without incurring in the above described drawbacks. In addition to the above, the fact of using humid air allows to have, under the same temperature, and because of the presence of humidity, an available thermal power greater than that of the case in which dry hot air is used.

In a further alternative embodiment, that is not shown in the figures, the hot air can be withdrawn both from the dry part 152 and from the humid part 151 of hood 150 and mixing the two flows of hot air, before feeding the hot air into the fixed hollow shaft 20.

The foregoing description exemplary embodiments of the invention will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiment without further research and without parting from the invention, and, accordingly, it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation. 

1. A Yankee dryer cylinder comprising: a fixed hollow shaft having a cavity and a lateral surface provided with a plurality of holes arranged to connect said cavity with the external environment, said fixed hollow shaft being provided with: an inlet mouth through which a flow of hot gas having a predetermined temperature, in particular a temperature higher than 100° C., is arranged to be fed into said cavity; an outlet mouth through which said flow of hot gas is arranged to exit said dryer cylinder; a cylindrical shell provided with an internal surface and rotatably mounted about said fixed hollow shaft, between the internal surface of said cylindrical shell and said lateral surface of said hollow shaft, an internal chamber being defined; said Yankee dryer cylinder further comprising: a partition wall mounted within said cavity in such a way to divide said fixed hollow shaft in a feeding portion and in a discharging portion, said feeding portion provided with at least one feeding hole arranged to feed said flow of hot gas from said cavity to said internal chamber, in such a way to hit said internal surface, said discharging portion provided with at least one discharging hole pneumatically connected to said outlet mouth, said flow of hot gas, after having hit said internal surface of said shell, being arranged to pass through said, or each, discharging hole, from said internal chamber to said cavity, in such a way to flow outside said Yankee dryer cylinder through said outlet mouth.
 2. The yankee dryer cylinder, according to claim 1, wherein a plurality of distributor members is provided that are housed within said internal chamber, each distributor member of said plurality arranged to distribute a share of said flow of hot gas, fed into said cavity of said fixed hollow shaft, on a respective determined portion of said internal surface of said shell.
 3. The yankee dryer cylinder, according to claim 2, wherein each distributor member of said plurality is tubular-shaped and is provided with at least one inlet aperture facing towards at least one feeding hole, and at least one distribution aperture facing towards said internal surface of said cylindrical shell.
 4. The yankee dryer cylinder, according to claim 3, wherein said, or each, distribution aperture is associated to a distribution plate provided with a plurality of distribution holes, in such a way to uniformly distribute the respective share of flow of hot gas on the respective portion of the internal surface of said cylindrical shell.
 5. The yankee dryer cylinder, according to claim 2, wherein said distributor members are arranged a at predetermined distance from each other, and said distribution apertures are arranged at a predetermined distance from said internal surface of said shell, in such a way that the lateral surfaces facing each other, of two next distributor members, are arranged to delimit a space, which, in use, is passed through by a flow of hot exiting gas directed towards said discharging holes.
 6. The yankee dryer cylinder, according to claim 1, wherein said feeding portion provides a plurality of feeding holes and said discharging portion provides a plurality of discharging holes, said feeding holes and said discharging holes being positioned on said fixed hollow shaft at different longitudinal rows.
 7. The yankee dryer cylinder, according to claim 1, wherein each distributor member of said plurality is oriented radially to said cylindrical shell, and extends longitudinally along said Yankee cylinder.
 8. The yankee dryer cylinder, according to claim 1, wherein said inlet mouth and said outlet mouth are provided at a same side of said fixed hollow shaft.
 9. The yankee dryer cylinder, according to claim 8, wherein, at the end of said fixed hollow shaft opposite to said inlet mouth and to said outlet mouth, a closing wall is provided.
 10. The yankee dryer cylinder, according to claim 1, wherein said partition wall is selected from the group consisting of: a partition wall arranged substantially orthogonal to the longitudinal axis of said fixed hollow shaft; a partition wall arranged longitudinal to said fixed hollow shaft; a partition wall, which extends longitudinally along said fixed hollow shaft (20), said partition wall comprising a first portion and a second portion inclined by a predetermined angle (α).
 11. The yankee dryer cylinder, according to claim 1, wherein said flow of hot gas is a flow of a mixture of gases.
 12. The yankee dryer cylinder, according to claim 1, wherein said flow of hot gas is a flow of hot air.
 13. The yankee dryer cylinder, according to claim 1, wherein said temperature of said flow of hot gas is set between 250° C. and 700° C., preferably set between 300° C. and 600° C.
 14. The yankee dryer cylinder, according to claim 1, wherein a suction device is provided connected to said outlet mouth of said fixed hollow shaft, said suction device arranged to suck said flow of hot gas fed into said cavity of said fixed hollow shaft through said inlet mouth.
 15. A plant for drying a humid web of paper, or similar product, comprising: the Yankee dryer cylinder, according to claim 1, said Yankee dryer cylinder provided with an external surface, on which, in use, said humid web to be dried is laid; a drying hood arranged, in use, at the opposite side of said Yankee dryer cylinder with respect to said humid web, said drying hood arranged to cause the drying of said humid web operating in combination with said Yankee dryer cylinder; a duct arranged to pneumatically connect said drying hood and said cavity of said fixed hollow shaft, said duct being arranged to feed a flow of hot gas circulating within said drying hood within said cavity.
 16. A method for drying a humid web of paper, or similar product comprising: disposing of a fixed hollow shaft having a cavity and a lateral surface provided with a plurality of holes arranged to connect said cavity with the external environment, said fixed hollow shaft provided with: an inlet mouth through which a flow of hot air is arranged to be fed into said cavity; an outlet mouth through which said flow of hot air is arranged to flow outside said dryer cylinder; disposing of a cylindrical shell provided with an internal surface and rotatably mounted about a said fixed hollow shaft, between the internal surface of said cylindrical shell and said lateral surface of said hollow shaft an internal chamber being defined; disposing of a partition wall mounted within said cavity in such a way to divide said fixed hollow shaft into a feeding portion and into a discharging portion, feeding a flow of hot air in said feeding portion, said flow of hot air passing from said cavity to said internal chamber through at least one feeding hole provided in said feeding portion, then, hitting said internal surface, and, then, passing in said discharging portion of said fixed hollow shaft through at least one discharging hole provided in said discharging portion, and, then, passing from said cylinder through said outlet mouth, said flow of hot air, after having hit said internal surface of said shell, being arranged to pass through said, or each, discharging hole from said internal chamber to said cavity, in such a way to flow outside said Yankee dryer cylinder through said outlet mouth. 